Sighting devices and devices of the type coming into consideration here have been known in practice for a long time and find a broad range of uses, in particular in the scope of contact-free temperature measurement. Devices for contact-free temperature measurement include in general a detector for the detection of a thermal radiation radiating from a measured spot on a measured object, an optical system for imaging the thermal radiation radiating from a measured spot on the detector, and a sighting device for identifying the position of the measured spot on the measured object by means of visible light. In addition to this, devices are known in which in addition to the position of the measured spot the size of the measured spot on a measured object is also made visible.
In practice, laser rays are used predominantly to make a measured spot on a measured object visible. In so doing however, a series of problems arises. If the laser, for example, is disposed to the side of the optical axis of the radiation detector and the laser ray is coupled at a small angle to the optical axis of the detector, then the laser sighting ray and the optical axis of the detector form two skew straight lines which intersect at a certain distance from the detector. As a consequence a sighting device of this type provides an error-free target marking only at a certain fixed distance of the measured object from the detector while for all other distances there is a more or less large discrepancy between the generated target marking and the actual measured spot.
The optics used in infrared measurement devices, in infrared thermometers as well as in infrared cameras, are often not permeable in the visible range. In order to obtain a precise distance-independent center display by means of laser sighting, constructions are necessary which contain, in the central area of the infrared optics on the optical axis of the infrared detector, reversing mirrors or deflecting mirrors as well as optical windows. It is common to all the constructions that they are quite elaborate, where in this connection let reference be made to U.S. Pat. No. 4,315,150 merely by way of example.
Due to the special processing of the infrared-optical materials the introduction of central holes in the objective is relatively expensive. The structural elements for measurement center sighting reduce the effective aperture of the infrared measurement channel due to their size. In addition to this, due to their self-radiation they represent, in particular in the case of changes of temperature in the device or object, an interfering variable which is difficult to compensate in the optical channel. Furthermore, diffraction phenomena occurring at the edges related to construction cause a reduction of the geometric resolution.